If you have a high level character already, buy the biggest bags you can and send them to your new character. This is a simple and easy to understand step that most people fail to use! If you have all 4 bag slots filled up right away, you can A) Continue questing without having to return to sell off items so you will level faster B) More importantly, be able to sell WOW Gold as you will not have to delete items to make room for others. You can sell all your loot when you return for maximum profit even from the very beginning!
For example, get herbalism and skinning or skinning and mining or mining and herbalism. Then, sell the loot you get! The Auction House is always alive with activity and buyers trying to get their hands on leather, ore and herbs. People are lazy, or they simply do not have the profession required by their trade skill! So, you arethe hero they needed to deliver the goods to them, and you pull a hefty profit at the same time!